The Princie Diamond, which has sold for $39.3 million at auction in New York City in 2013.

The heirs of an Italian senator sued Christie’s International Plc over the sale of a rare diamond valued at more than $40 million that they say was stolen.

Christie’s failed to investigate the rightful ownership of the “Princie Diamond” before auctioning it off in 2013, Senator Renato Angiolillo’s heirs said in the complaint. Christie’s rejected the accusation, blaming the family for fighting over an inheritance.

The 34.65 carat diamond, mined more than 300 years ago in India and named after a prince, is maybe the most famous and illustrious pink diamond in the world, according to the complaint.

The ownership dispute began after Angiolillo’s second wife died in 2009. The senator’s heirs asked her son to return the diamond and other gems, which his wife was allowed to hold following his death in 1973. They were told she didn’t have the gems, according to the complaint.

The heirs began a search for the missing gems and Italian authorities began a criminal investigation, according to the lawsuit. In early April 2013, the heirs learned that Christie’s planned to sell the Princie Diamond later that month.

They said they contacted the auction house and asked it to investigate the diamond’s origin. Christie’s refused, saying the ownership of the diamond was properly transferred under Swiss law, and allowed the sale to go ahead, collecting a $3.8 million commission, according to the complaint.

“New York law provides that once there is a ‘thief,’ an involuntary transfer, or the property is lost in the chain of title, no one can take good title,” the heirs said in the complaint.

Christie’s said in a statement that the suit “stems from a long-standing and highly contentious inheritance dispute among the members of an Italian family.”

“Prior to Christie’s sale of the diamond in 2013, the two main representatives of the family had expressly withdrawn any objection to the sale,” Christie’s said. “They are well aware that the consignor of the Princie Diamond was not a member of the family, but a bona fide purchaser who acquired the diamond in an arms-length transaction for a significant sum and had held full and clear title to it for several years.”